This invention pertains to color photograph printers and enlargers and more particularly to subtractive-printing-type lamp houses.
In the usual subtractive printer, a plurality of filters are inserted between a source of white light and a color negative or transparency which is to be printed. Typically, three filters, magenta, cyan and yellow are used. Also typical, the filters are each mounted in a slidable carrier frame, all of such frames being adjacent an aperture and slidable to block a selected portion of the aperture. Thus, by positioning of the frames with respect to the aperture, a desired light mix may be obtained for printing the photograph.
The amount and type of filtration of the white light determines the colors in the final photograph. Accordingly, the positioning of each filter with respect to the aperture is a critical function of every color printer. Such positioning must be accurate and there must also be provision for accurate return to prior positions so that previously obtained color densities in photographs may be reproduced. To complicate matters, there is a logarithmic relationship between the position of each filter with respect to the aperture and the density of its associated color which appears in the final print. In other words, when one filter is moved from a position fully exposing the aperture to a position fully spanning the aperture, the decrease in light intensity as the filter first blocks the aperture is much smaller, for a given amount of movement, than when such movement occurs when the filter has nearly spanned the aperture. In the past, each filter has been positioned by a non-linear cam, such cam either moving the filter carrier toward a position spanning the aperture or toward a position fully exposing the aperture depending upon the direction of rotation of the cam. The cam is machined to correspond to the above-described logarithmic relationship so that equal amounts of rotation produce equal amounts of light intensity change regardless of the position of the filter with respect to the aperture.
In the past, such logarithmic cams have been hand operated as opposed to motor operated due to the critical positioning required for the filters. Even so, such prior art positioning does not allow precision placement of the filters with respect to the aperture so that exact light intensities for each filter may be obtained. Another problem in the prior art is that once a desired position is obtained, it is extremely difficult to reposition the filters to the desired position after they have been moved.
According to a preferred embodiment of the instant invention, the above-described filter carriers are positioned by stepper motors, each driving a screw shaft, such being interconnected with its associated filter carrier. Each stepper motor (and therefore each filter) is under the control of a minicomputer. Stored in the computer's memory is what might be thought of as a "look-up table" for each motor. Such table includes a plurality of density point values, each such value being associated with a number of steps required to drive the motor until the filter is positioned to generate the density point value for its associated color. Additionally, a photosensor, one for each filter, may be placed in the path of the mixed light. The output of each photosensor is converted to a digital signal and fed back to the computer. Such feedback is used to insure that when a user selects a specific density point value for a certain filter, that such density point value will be achieved.
Thus, a general object of the invention is to provide accurate color filtering position in a color printing device.
A more specific object of the invention is to provide such positioning with the use of sensor feedback which samples the intensity of the light associated with each filter.
Another object of the invention is to provide exact repositioning of the filters, so that a given filter combination may be exactly reproduced when the user so desires.
These and other objects and advantages of the invention will become more fully apparent as the description which follows is read in conjunction with the accompanying drawings.